This invention relates to an ion source apparatus which generates plasma by means of gas discharge and which acceleratedly extracts ions from said plasma.
The conventional ion source apparatus for supplying ions to a neutral beam injector or ion implantation apparatus comprises a thermionic cathode composed of tungsten filaments, an anode, and a grid electrode to acceleratedly extract ions from plasma generated by a gas discharge occurring between a cathode and an anode. The conventional ion source apparatus arranged as described above has the drawbacks that the thermionic cathode composed of tungsten filaments loses its flesh to snap off because the cathode is heated up to high temperature and suffers from the bombardment of ions included in the plasma. Consequently, the thermionic cathode has to be frequently replaced by another one due to its short life.
To resolve the above-mentioned difficulties accompanying the conventional ion source apparatus, another ion source apparatus has been put to practical use in which a bulk cathode shaped, for example, like a disk is applied in place of the filament type of thermionic cathode. With this proposed ion source apparatus, the anode surface is surrounded by permanent magnets or electromagnets in order to effectively confine the plasma generated by the gas discharge, thereby efficiently producing ions. Such a magnet is indeed effective in confining plasma, but it is accompanied with the drawbacks that a magnetic field covers the surface of the bulk thermionic cathode to obstruct the electron emission of the cathode, thereby obstructing the gas discharge.